A method and apparatus for controlling an actuator of a crane by control means in a situation where the lifting power of the actuator of the crane is temporarily increased by means of an auxiliary valve arrangement.
Transport vehicles, for example log trucks and various lumbering machines, are equipped with loading cranes, the main purpose of which is to move, load or unload a load or perform other similar measures. A loading crane may also be utilized in other tasks essentially related to the work, where a heavy load is moved to improve working conditions or to reduce work-related expenses, an example of which is the avoidance of different road taxes dependent on the length of an articulated vehicle by lifting, for instance, a semi-trailer onto the cargo space of the vehicle body when there is no actual transportable load in the cargo spaces of the vehicle or its trailer. By lifting the semi-trailer onto the cargo space on top of the vehicle body, the length of the vehicle becomes essentially shorter and the road tax is lower when the vehicle is transported on a road. Depending on the weight of the semi-trailer, it is often necessary to temporarily increase the lifting power of the crane when the semi-trailer is lifted onto the vehicle body. Since the lifting power can be increased temporarily, it is thus possible to avoid the purchase of a crane having a higher lifting power and being thus heavier and more expensive in terms of both the purchase price and the operating costs only because the increased lifting power is required temporarily.
The design of loading crane constructions is based on standards, which define the calculation basis for the structures of mechanical parts according to the desired lifting power, load and work rotations, lifting class, load group and method of application, for instance. The calculation basis also includes dynamic coefficients. Dynamic coefficients define, for instance, lifting power and gravitational force effects of the crane parts and the load, i.e. the total load, and effects of total load acceleration or deceleration. The dynamic coefficient thus affects the lifting class of the crane, which, in turn, affects material selections and other cost factors associated with crane manufacture. The manufacture. The service life of the crane is affected by stress accumulations directed at the crane structures and formed during loading. The stress accumulation is in practice influenced by the static maximum stress level during the crane operation, which, in this case, is defined on the basis of the hydraulic operating pressure used in the crane, and by dynamic stress peaks occurring during the operation, which are due to accelerations or decelerations of the total load. The method and apparatus of the invention may affect the stress accumulation during load and work rotations in such a manner that the service life does not become essentially shorter, although the lifting power is temporarily increased. This property may be utilized during loading in situations where the normal lifting power of the crane is not sufficient for lifting a big load but there is a need for temporarily increasing the lifting power, whereupon the possibility to temporarily increase the lifting power of the crane without essentially shortening the service life of the crane allows to avoid the purchase of a bigger and thus more expensive and heavier crane.
In known solutions, to solve the above problem there is provided a method and a control apparatus, in which there is a separate actuator-specific pressure relief valve for increasing the lifting power in a pressure medium space on the operation side, i.e. on the piston side, of the lifting cylinder. A separate pressure relief valve is adjusted to an actuator-specific pressure level determined by normal pressures, i.e. normal lifting power. Likewise, said separate pressure relief valve is provided with a directional control valve, which may be controlled electrically to provide the actuator with a higher pressure level, if desired. The control apparatus of the crane also comprises the crane's actual control valve, the piston side of the lifting cylinder of which comprises an actuator-specific pressure relief valve, which is adjusted to the pressure level determined by the increased lifting power. By setting the separate directional control valve to an open position, the pressure level of the actuator-specific, separate pressure relief valve is determined as decisive, in this case as equivalent to the normal pressure level. By setting the separate directional control valve to a closed position, the actuator-specific pressure level is determined to have the pressure level determined by the actuator-specific pressure relief valve of the actual control valve, which in this case corresponds to the increased lifting power. In addition to the above arrangement, the hydraulic circuit of the crane is provided with a bypass flow control valve in a pressure line between a pump and the actual control valve in such a manner that an control valve in such a manner that an amount of the pump output preset in the bypass flow control valve may be guided electrically directly to a return line of the pressure medium. This arrangement aims at lowering the crane's speed of motion in cases where the crane is driven with the increased lifting power. The objective has been to reduce stress peaks caused by accelerations and decelerations of steering movements by lowering the crane's speed of motion. In addition to the above, the hydraulic circuit of the crane is provided, in the pressure line between the pump and the actual control valve, with a separate main-pressure relief valve, which helps to determine the maximum pressure level for the entire hydraulic circuit of the crane. The separate main-pressure relief valve is adjusted to a pressure level determined by normal pressures, i.e. the normal lifting power. In connection with the separate main-pressure relief valve there is also provided a directional control valve, which may be electrically controlled when the crane should be provided with a higher pressure level. In connection with the actual control valve of the control apparatus of the crane there is a main-pressure relief valve, which is adjusted to a pressure level determined by the increased lifting power. By setting the directional control valve in connection with the separate main-pressure relief valve to an open position, the pressure level of the separate main-pressure relief valve is determined as decisive, in this case as equivalent to the normal pressure level. By setting the directional control valve to a closed position, the pressure level of the crane is determined to have the pressure level determined by the main-pressure relief valve of the actual control valve, which in this case corresponds to the increased lifting power. Both above-mentioned directional control valves are controlled synchronously, whereby the pressure level determined by the separate, actuator-specific main-pressure relief valve and that determined by the separate main-pressure relief valve correspond to one another.
A problem with the above-mentioned implementation is that stress peaks of the structures due to accelerations or decelerations of the total load during the crane operation are particularly caused by pressure peaks occurring on the piston side of the lifting cylinder. The most significant factor in causing pressure peaks particularly during the lowering of the load is the design of the guide edges of the spindle of the actual control valve, particularly when it comes to the spindle part determining the control properties when the pressure medium is guided from the piston side of the lifting cylinder along the return line to the tank. In the above-mentioned implementation, the bypass flow control valve does not affect the pressure medium flowing from the piston side of the lifting cylinder to the return line, which means that it does not either affect the speed at which the load is lowered downwards or its deceleration or the stress peaks higher than normal pressure, which are due to the acceleration or deceleration caused by the increased pressure level and the corresponding load, whereby the service time of the crane also becomes shorter.
There are also systems, in which the increased lifting power is implemented by means of control electronics and sensors of the crane. Patent WO9319000 discloses an implementation, in which the pressure of the operation side of a lifting cylinder is monitored by a pressure sensor. On the basis of signals of the pressure sensor and an angle sensor mounted in a boom arrangement, software controls the components of the crane hydraulic system according to a certain logic and provides an increased pressure level and reduced speeds of motion for the actuators of the crane, when the conditions defined in the software are fulfilled.
A problem with the above implementation is that the apparatus requires a lot of electronics, sensors and other equipment necessary for building an electronic apparatus. Consequently, the system is expensive in terms of both a purchase price and maintenance costs. An electronic implementation is also susceptible to faults when compared with a mechanical system, in which hydraulic components are controlled by simple electrotechnics.